Joseph G. Allen is an associate professor and director of the Healthy Buildings program at Harvard University’s T.H. Chan School of Public Health. Akiko Iwasaki is a professor of immunobiology at Yale School of Medicine. Linsey C. Marr is a professor of civil and environmental engineering at Virginia Tech.
Fortunately, we are not helpless in this fight. We have a great set of tools that can help slow the spread of this virus: masks, social distancing and hand-washing, as well as healthy building strategies such as ventilation and filtration.
And there is one more healthy building tool that we can use this winter: maintaining relative humidity in the 40-to-60-percent range.
Relative humidity is the term for how much water vapor is actually in the air compared to how much it can hold. Think of it like a sponge: At 100 percent, the sponge is totally soaked; at 50 percent, it holds half as much water. Warmer air can hold more water vapor; it’s like a bigger sponge. As fall turns to winter and we start heating the air, our indoor environments become more dry, often hitting 20 percent relative humidity, well below the ideal 40 to 60 percent.
Humidity can affect transmission in three ways. First, it influences our body’s ability to fight off infection. The first line of defense in the lungs relies on the combined effect of mucous in the respiratory tract and the upward beating of the hairlike cilia on lung cells. The sticky mucous acts to catch particles, including viruses, as they descend into the lungs. Once trapped, the cilia push the mucous and particles up to the throat, where they are swallowed harmlessly. It’s a “mucociliary escalator,” and it works great — most of the time.
One problem: In dry air, there is less mucous, and the cilia don’t beat as fast or in the right direction, as new research shows. This means fewer virus particles are captured or cleared out of the respiratory tract, thus allowing more of them to reach the deepest part of our lungs, where they do the most damage.
Second, a new study shows that the coronavirus decays faster at close to 60 percent relative humidity than at other levels. While this study has not yet been peer-reviewed, there is a trove of work showing that other viruses also decay faster in the range of 40 to 60 percent. We don’t exactly know why this is the case, but we might as well take advantage of the knowledge and avoid dry conditions.
Third, dry air also influences how far droplets containing the virus can travel and how long they can stay in the air. When we talk, sing or breathe, we emit respiratory droplets. Some are quite large and fall to the ground quickly. Many more are microscopic — we call them “aerosols” — and they stay aloft in the air for hours and travel well beyond six feet.
Here’s the problem: The lower the relative humidity, the faster those larger droplets evaporate. The result is that many of those larger droplets that would have settled out of the air turn into the peskier smaller ones that travel farther and penetrate deeper into our lungs. This effect is smaller than the other two mentioned above, but it still favors higher humidity to reduce the risk of transmission.
If higher humidity is good, why don’t we want to go higher than 60 percent? Mold growth. A level of 40 to 60 percent is right in the sweet spot where we get the benefits of some humidity but not the drawbacks of too much.
We’re about to spend a lot more time inside as it gets colder, and indoor spaces are where nearly all transmission is happening. In the winter, we bring air indoors and heat it. We increase the size of the sponge without adding more water vapor. Therefore, the indoor relative humidity ends up being lower than the outdoor humidity.
Unfortunately, there’s not a lot most public or commercial buildings can do about it. Most existing HVAC systems — even at hospitals — don’t have humidification built in, and it’s not necessarily an easy fix.
In a home, however, you can use portable humidifiers to increase the relative humidity in a room. For example, a study of influenza transmission found that using portable humidifiers in a home could result in up to 30 percent lower influenza virus survival. Given that transmission is occurring frequently within the home, humidification could be an important additional control strategy. (Just remember to follow the manufacturer’s recommendations on maintenance. Any time you introduce water into a building, you run the risk of creating damp conditions that are ripe for mold growth.)
As winter sets in, we need to defend our homes, schools and offices. In addition to other strategies — masks, distancing, ventilation and filtration — raising the humidity is another layer of defense to consider.
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